1. Field of the Invention
The subject invention relates to a knee joint prosthesis having enhanced valgus-varus stability.
2. Description of the Prior Art
A natural knee joint includes the distal end of the femur, the proximal end of the tibia and a meniscus bearing therebetween. The femur and the tibia are held in proper relationship to one another and to the bearing by a plurality of ligaments, including the posterior cruciate ligament, the anterior cruciate ligament and collateral ligaments. Flexion of the knee joint causes the tibia to rotate relative to the femur about an axis extending generally in a medial-to-lateral direction. Flexion also generates rotation of the tibia about its own axis.
Damage or disease can affect the ability of the natural knee to function properly. The damage or disease can deteriorate the bones, the articular cartilage, the ligaments or some combination thereof. A damaged or diseased natural knee can be replaced by a prosthetic knee joint. A prior art knee joint prosthesis includes a femoral component securely mounted to the distal end of a resected femur, a tibial component securely mounted to the proximal end of a resected tibia and a bearing disposed between the femoral and tibial components. The inferior face of the femoral component includes a pair of condyles. The condyles have a convexly arcuate shape, and the superior surface of the bearing has a pair of arcuate concave regions for articular bearing engagement with the condyles of the femoral component. The superior face of the tibial component may be substantially planar and is in bearing engagement with the inferior face of the bearing.
Currently available prosthetic knee joints take many different forms depending upon the preferences of the orthopedic surgeon, the condition of the natural knee and the health, age and mobility of the patient. Some prior art knee joint prostheses fixedly mount the inferior surface of the bearing to the superior surface of the tibial component. Other knee joint prostheses permit rotary movement and/or sliding movement between the bearing and the tibial component. Movement of the bearing against the tibial component achieves many functional advantages described in the prior art. These functional advantages include an avoidance of dislocation in response to normal walking movement without reliance upon a fixed hinged connection. Very effective prior art knee joint prostheses that incorporate certain of the structural features referred to herein are disclosed in U.S. Pat. Nos. 4,470,158 and 4,309,778.
Valgus is a Latin term which translates roughly as bowlegged or knock-kneed. Varus also is a Latin word and translates roughly as crooked. The Latin word varus often is used to define an abnormal position of a bone of the leg or foot. Valgus-varus stability of a knee joint refers to the ability of the joint to resist lateral forces or rotary forces that would tend to urge one knee toward or away from the other. In a knee joint prosthesis, lateral forces or rotary moments that would tend to urge one knee toward or away from the other will also tend to create a dislocation particularly on one side of the prosthesis or the other, as shown, for example, in the prior art prosthesis of FIG. 14.
During normal activities, and with the prosthetic knee joint under compressive loading, the valgus-varus moments are resisted primarily and adequately by the articulating surfaces and ligaments. However, there may be certain instances, for example where ligaments are deficient, when additional valgus-varus stability may be desired.
Some prior art prosthetic knee joints have improved valgus-varus stability by providing a stabilizing post that extends into the posterior region between the femoral condyles that would be occupied by the posterior cruciate ligament if the posterior cruciate ligament were present. For example, U.S. Pat. No. 5,395,401 to Bahler shows a prosthetic knee having a tibial platform and a bearing slidably disposed on the tibial platform. The inferior surface of the bearing is provided with a dove-tailed groove that extends substantially in an anterior-posterior direction and at a location between the two concave condyles formed on the superior surface of the bearing. The bearing shown in U.S. Pat. No. 5,395,401 further includes a notch extending into the posterior side of the bearing at a location between the two concave condyles of the bearing. Thus, the notch registers with the dove-tailed groove of the bearing. The prosthesis of U.S. Pat. No. 5,395,401 further includes a control arm. The control arm includes a post that is pivotally engaged in a recess of the tibial component. The control arm also includes a dove-tailed portion that slidably engages in the dove-tailed groove on the inferior surface of the bearing. Furthermore, the control arm of U.S. Pat. No. 5,395,401 include a post that extends through the notch in the bearing and between the condyles of the femoral component. The post is dimensioned to slidably engage surfaces of the femoral component between the two convex condyles of the femoral component. Thus, the post of the control arm will resist bending moments created by medial or lateral forces, and hence will provide enhanced valgus-varus stability.
Other prior art prosthetic components have provided enhanced valgus-varus stability with posts that extend unitarily from the bearing and into the space between the femoral condyles. Examples of such prior art prosthetic knees are shown, for example, in U.S. Pat. Nos. 5,658,342; 5,489,311; 5,330,534; 4,950,298; 4,888,021, 4,634,444 and 4,568,348. All of these prior art prosthesis are used for joint replacements where the posterior cruciate ligament cannot be retained or are deficient. Additionally, most of these prior art prosthesis are used where both collateral ligaments can be retained. The retained collateral ligaments cooperate with the post to resist valgus-varus moments and to prevent dislocation. Prior art prosthetic joints that have a post extending into the posterior notch between the femoral condyles have not been constructed to both resist valgus-varus moments and to substantially prevent dislocation in instances where only one collateral ligament can be retained.
Accordingly, it is an object of the subject invention to provide a prosthetic knee joint having an enhanced valgus-varus stability.
It is a further object of the subject invention to provide enhanced valgus-varus stability in a prosthetic knee joint without requiring a hinged connection.
It is an additional object of the subject invention to provide a prosthetic knee joint with enhanced protection against dislocation without requiring a hinged connection.
It is yet another object of the subject invention to provide axial rotation without a control arm, and thereby simplifying the design of the prosthetic knee.